Historically, today's most favorite rechargeable energy storage devices—lithium-ion batteries—actually evolved from rechargeable “lithium metal batteries” using lithium (Li) metal as the anode and a Li intercalation compound as the cathode. Li metal is an ideal anode material due to its light weight (the lightest metal), high electronegativity (−3.04 V vs. the standard hydrogen electrode), and high theoretical capacity (3,860 mAh/g). Based on these outstanding properties, lithium metal batteries were proposed 40 years ago as an ideal system for high energy-density applications. During the mid-1980s, several prototypes of rechargeable Li metal batteries were developed. A notable example was a battery composed of a Li metal anode and a molybdenum sulfide cathode, developed by MOLI Energy, Inc. (Canada). This and several other batteries from different manufacturers were abandoned due to a series of safety problems caused by sharply uneven Li growth (formation of Li dendrites) as the metal was re-plated during each subsequent recharge cycle. As the number of cycles increases, these dendritic or tree-like Li structures could eventually traverse the separator to reach the cathode, causing internal short-circuiting.
To overcome these safety issues, several alternative approaches were proposed in which either the electrolyte or the anode was modified. The first approach involved replacing Li metal by graphite (another Li insertion material) as the anode. The operation of such a battery involves shuttling Li ions between two Li insertion compounds, hence the name “Li-ion battery. Presumably because of the presence of Li in its ionic rather than metallic state, Li-ion batteries are inherently safer than Li-metal batteries. The second approach entailed replacing the liquid electrolyte by a dry polymer electrolyte, leading to the Li solid polymer electrolyte (Li-SPE) batteries. However, Li-SPE has seen very limited applications since it typically requires an operating temperature of up to 80° C.
The past two decades have witnessed a continuous improvement in Li-ion batteries in terms of energy density, rate capability, and safety, and somehow the significantly higher energy density Li metal batteries have been largely overlooked. However, the use of graphite-based anodes in Li-ion batteries has several significant drawbacks: low specific capacity (theoretical capacity of 372 mAh/g as opposed to 3,860 mAh/g for Li metal), long Li intercalation time (e.g. low solid-state diffusion coefficients of Li in and out of graphite and inorganic oxide particles) requiring long recharge times (e.g. 7 hours for electric vehicle batteries), inability to deliver high pulse power (power density <<1 kW/kg), and necessity to use pre-lithiated cathodes (e.g. lithium cobalt oxide), thereby limiting the choice of available cathode materials. Further, these commonly used cathodes have a relatively low specific capacity (typically <200 mAh/g). These factors have contributed to the two major shortcomings of today's Li-ion batteries—a low energy density (typically 150-180 Wh/kgcell) and low power density (typically <0.5 kW/kg).
Although several high-capacity anode active materials have been found (e.g., Si with a theoretical capacity of 4,200 mAh/g), there has been no corresponding high-capacity cathode material available. To sum it up, battery scientists have been frustrated with the low energy density of lithium-ion cells for over three decades!
Current cathode active materials commonly used in Li-ion batteries have the following serious drawbacks:                (1) The practical capacity achievable with current cathode materials (e.g. lithium iron phosphate and lithium transition metal oxides) has been limited to the range of 150-250 mAh/g and, in most cases, less than 200 mAh/g.        (2) The production of these cathode active materials normally has to go through a high-temperature sintering procedure for a long duration of time, a tedious, energy-intensive, and difficult-to-control process.        (3) The insertion and extraction of lithium in and out of these commonly used cathodes rely upon extremely slow solid-state diffusion of Li in solid particles having very low diffusion coefficients (typically 10−8 to 10−14 cm2/s), leading to a very low power density (another long-standing problem of today's lithium-ion batteries).        (4) The current cathode materials are electrically and thermally insulating, not capable of effectively and efficiently transporting electrons and heat. The low electrical conductivity means high internal resistance and the necessity to add a large amount of conductive additives, effectively reducing the proportion of electrochemically active material in the cathode that already has a low capacity. The low thermal conductivity also implies a higher tendency to undergo thermal runaway, a major safety issue in lithium battery industry.        (5) The most commonly used cathodes, including lithium transition metal oxides and lithium iron phosphate, contain a high oxygen content that could assist in accelerating the thermal runaway and provide oxygen for electrolyte oxidation, increasing the danger of explosion or fire hazard. This is a serious problem that has hampered the widespread implementation of electric vehicles.        
For use in a rechargeable lithium metal battery (i.e. a secondary battery using lithium metal as an anode-active material), the chalcogenide is the most studied cathode-active material. The chalcogenide is formed of the sulfides, selenides or tellurides of titanium, zirconium, hafnium, niobium, tantalum, or vanadium. A largely overlooked class of cathode active materials is phthalocyanine. There was an earlier attempt to use phthalocyanine-based cathode in a lithium metal battery [J. Yamaki and A. Yamaji, “Phthalocyanine cathode materials for secondary lithium cells,” Electrochemical Society Journal, vol. 129, January 1982, p. 5-9; J. Yamaki and A. Yamaji, U.S. Pat. No. 4,251,607, Feb. 17, 1981]. In addition to the aforementioned dendrite problem, these cathodes (both chalcogenide and phthalocyanine) and related lithium metal batteries suffer from many major issues:                (a) These cathode active materials are electrically insulating and, hence, require the use of a large amount of conductive additives (e.g. carbon black, CB, or acetylene black, AB) that are electrochemically inactive materials (not contributing to lithium storage, yet adding extra weights to the cell). For instance, in Yamaki et al (1982) cited above, for every 0.1 grams of metal phthalocyanine, 0.1 grams of acetylene were added. With another 10% by weight of a resin binder, the proportion of the cathode active material alone (phthalocyanine itself) in the cathode is less than 50% by weight.                    By plotting the cathode specific capacity data listed in Table 1 of Yamaki, et al (1982) we obtained FIG. 1(A), which indicates that the lithium storing capacity per gram of the cathode active material only (hydrogen phthalocyanine, H2Pc) actually decreases with the increasing proportion of the active material amount (or decreasing acetylene black proportion). It is very disturbing that at least 50% by wt. of AB is required. If the weight of acetylene black (AB, a conductive additive) is accounted for, the cathode specific capacity is down to unacceptable values of 71.8-345 mAh/g (of the H2Pc and AB weights combined, not counting the resin binder weight), as indicated in FIG. 1(B). These are much lower than what can be achieved with the theoretical capacity (800-900 mAh/g) of H2Pc.                        (b) These lithium metal cells exhibit very poor rate capability. In other words, their lithium storing capacity drops significantly when a higher charge/discharge rate or higher current density is imposed on the cells. Table 2 of Yamaki, et al (1982) indicates that the specific energies of manganese phthalocyanine (MnPc), iron phthalocyanine (FePc), cobalt phthalocyanine (CoPc), and nickel phthalocyanine (NiPc) based on the active material weight alone were 2240, 2300, 1530, and 2220 Wh/kg (of active material weight), respectively, when the discharge current density was at 1 mA (or 5 mA/g based on the combined metal Pc/AB weight of 0.2 g). When the discharge current was increased to 3.14 mA for 0.2 g (or 15.7 mA/g, still a very low discharge rate), the corresponding specific energies dropped to 430, 730, 410, and 370 Wh/kg (of active material weight only), respectively. By dividing these energy density values by a factor of 5, one obtains the estimated cell-level energy densities of 86, 146, 82, and 74 Wh/g that are much lower than those of current lithium-ion cells. These are unacceptably low for consumer electronics, power tool, renewable energy storage, and electric vehicle power applications.        (c) These cells are not very reversible and typically have very poor cycling stability and short cycle life. For instance, according to FIG. 10 of Yamaki, et al (1982), most of the cathode specific capacity dropped to an unacceptably low vale in less than 30 cycles (the best was only 100 cycles, for Cu phthalocyanine).        (d) Most of these cathode active materials are slightly soluble in the liquid electrolyte, gradually losing the amount of cathode active material available for lithium storage. This is more severe for phthalocyanine compounds wherein the anions are highly soluble in commonly used lithium cell electrolytes (e.g. metal phthalocyanine has high solubility below 1 volt vs. Li/Li+). This is one major reason why the cycling stability of these cells is so poor.        (e) All the metal phthalocyanine compounds (MPc) have a catalytic effect on decomposition of electrolytes, creating cycle reversibility and long-term stability issues.        
Thus, it is an object of the present invention to provide a phthalocyanine compound-based high-capacity cathode active material (preferably with a specific capacity much greater than 300 mAh/g) for use in a secondary lithium cell (either lithium metal cell or lithium-ion cell) having a long cycle life.
It is another object of the present invention to provide a rechargeable lithium cell featuring a phthalocyanine compound-based high-capacity cathode active material exhibiting a cathode specific capacity greater than 500 mAh/g, typically greater than 1,000 mAh/g, preferably greater than 1,500 mAh/g, or even greater than 2,100 mAh/g.
It is still another object of the present invention to provide a high-capacity cathode active material (with a specific capacity significantly greater than 300 mAh/g, up to 2,200 mAh/g) that can be readily prepared without going through an energy-intensive sintering process.
Another object of the present invention is to provide a high-capacity cathode active material (with a specific capacity greater than 300 mAh/g or even greater than 2,100 mAh/g) that is amenable to being lithium intercalation-free or fast lithium intercalation, leading to a significantly improved power density.
Yet another object of the present invention is to provide a high-capacity cathode active material that is electrically and thermally conductive, enabling high-rate capability and effective heat dissipation.
It is still another object of the present invention to provide a high-capacity cathode active material that contains little or no oxygen, reducing or eliminating the potential fire hazard or explosion.
Still another object of the present invention is to provide a rechargeable lithium cell that has a long charge-discharge cycle life (>300 cycles, preferably >500 cycles, and most preferably >1,000 cycles) and has a phthalocyanine compound-based high-capacity cathode active material that is not significantly soluble in the electrolyte used.
It is an ultimate object of the present invention to provide a high energy density, rechargeable lithium cell that features a high-capacity cathode active material and exhibits an energy density significantly greater than the best of existing Li-ion cells.